Music programs and the like are distributed remarkably nowadays by means of wired or radio broadcast or communication. For the distribution of music programs and the like, it is important to prevent a music data amount from becoming large and an occupied band width from broadening, if the band width is made too broad. To avoid this, music data is distributed after it is compressed by a voice compression format incorporating a frequency masking method, such as an MP3 (MPEG1 audio layer 3) format and an AAC (Advanced Audio Coding) format.
The frequency masking method is a method of compressing voices by utilizing the phenomenon that a human being is hard to hear the spectrum components of a low level sound signal whose frequency is near the spectrum components of a high level sound signal.
FIG. 4(b) is a graph showing the results of compressing an original sound spectrum shown in FIG. 4(a) by using the frequency masking method (FIG. 4(a) shows an example of the spectrum obtained by compressing voices produced by a human being by the MP3 format).
As shown, as the voices are compressed by the frequency masking method, generally the components having a frequency of 2 kHz or higher are lost considerably, and the components even lower than 2 kHz near the components providing a spectrum peak (spectrum of a fundamental frequency components and harmonic components of voices) are also lost considerably.
A method disclosed in Japanese Patent Laid-open Publication No. 2001-356788 interpolates a compressed voice spectrum to obtain an original voice spectrum. According to this method, an interpolation band is derived from the spectrum left after the compression and the spectrum components indicating the same distribution as that in the interpolation band are inserted into the band whose spectrum components were lost by the compression, so as to match the envelope line of the whole spectrum.
If the spectrum shown in FIG. 4(b) is interpolated by the method disclosed in the Japanese Patent Laid-open Publication No. 2001-356788, the spectrum shown in FIG. 4(c) is obtained which is quite different from the spectrum of the original voices. Even if the voices having this spectrum are reproduced, only very unnatural voices are obtained. This problem is generally associated with voices produced by a human being and compressed by this method.
The present invention has been made under the above-described circumstances and it is an object of the invention to provide a frequency interpolation apparatus and method for recovering voices of a human being from the compressed voices while maintaining a high sound quality.